1. Field of the Invention
The present invention relates to a semi-transmissive liquid crystal display device having a light reflective region and a light transmissive region in a single pixel region, and more particularly, to the structure of a spacer that defines an interval between a pair of top and bottom substrates.
2. Description of Related Art
In recent years, a liquid crystal display device has been used extensively as a large-scale, high-definition monitor besides a small- or medium-scale personal digital assistant and a notebook computer.
A display mode of the liquid crystal display device includes a reflective mode in which display is performed using external light that comes incident on the display surface, a transmissive mode in which a backlight is used and display is performed by allowing light from the backlight to pass through the display surface, and a semi-transmissive mode furnished with the functions of both modes.
A semi-transmissive liquid crystal display device achieving the semi-transmissive mode is used as a reflective device with the use of external illumination, such as sunlight or fluorescent light, or is used as a transmissive device by attaching a backlight. In order to provide the display functions of both types, it uses a semi-transmissive film (structured to serve as a half mirror by laminating plural dielectric films having different refractive indexes alternately).
When such a semi-transmissive film serving as a half mirror is used, it is difficult to enhance both functions of reflectance and transmittance.
In order to solve this problem, a semi-transmissive liquid crystal display device is assembled by forming a reflection film in a pixel region and providing a light transmission hole (light transmissive region) in part of the reflection film formed in the pixel region for allowing light from the backlight to pass through.
Meanwhile, it is quite important for the liquid crystal display device to keep a liquid crystal layer uniform at a thickness of a specific value. Hence, in general, a number of fine, transparent spacers to keep the liquid crystal layer at a uniformed thickness are mixed in the liquid crystal layer.
For this reason, in a semi-transmissive liquid crystal device, for example, when black is displayed in a transmissive mode (a display state where transmitted light is shielded), white dropouts readily occur due to the transparent spacers per se and an alignment defect of the liquid crystal on the periphery of the spacers. This is because while liquid crystal molecules are in a twisted state in a space between two substrates, those on the periphery of the spacers go into a state where they apparently stand perpendicularly in the space between the two substrates, and the twisted state of the liquid crystal molecules on the periphery of the spacers is thereby disturbed, which gives rise to a light leaking phenomenon in the liquid crystal layer near the spacer surface.
In addition, spacers provided in a space between two substrates are transparent, and provided on a random basis by means of wet scattering or dry scattering. Hence, the number of spacers provided above the light transmission hole is not constant.
Because a degree of white dropouts in each pixel depends on the number of spacers provided above the light transmission hole, white dropout portions across the entire display region appear as irregularities.
It is possible to prevent the white dropouts caused by a defect of liquid crystal alignment as described above by using spacers (hydrophilic spacers) having undergone treatment to form alkyl groups on the spacer surface. However, because only part of the pixel contributes to a transmissive display in the transmissive mode, the size of the pixel becomes relatively small with respect to the size of the spacers. Hence, when white is displayed on the contrary (a display state where transmitted light is allowed to pass through), black irregularities, by contrast, appear across the entire display region when an alignment restraining force is too strong.
In the semi-transmissive liquid crystal display device having both the light reflective region and the light transmissive region in the pixel region, display irregularities occur after all either in the black display or in the white display.
When the liquid crystal display device is formed using the hydrophilic spacers, leakage of light from the periphery of the spacers can be prevented. However, there arises another problem that an alignment defect of liquid crystal molecules occurs between close spacers.
In order to keep the liquid crystal layer uniform, the necessary mixing density of spacers is 80 or more pieces per 1 mm2 when viewed in a plane, and in order to maintain the liquid crystal layer in a stable manner, a density of 200 pieces per 1 mm2 is necessary. However, when hydrophilic spacers are mixed at a high density, an alignment defect of liquid crystal molecules between spacers becomes noticeable.
FIG. 6 is a plan view of pixel regions showing an inconvenience caused by a defective liquid crystal alignment between hydrophilic spacers.
Hydrophilic spacers 10a are provided in the respective pixel regions. The base material of the spacers is formed by subjecting a monomer having unsaturated ethylene groups to suspension polymerization using a radical polymerizing agent. Polymerization treatment to polymerize alkyl groups (having about 12 carbons) on the surface of the base material of the spacers is applied as hydrophilic treatment.
As a consequence, when the spacers 10a to surfaces of which is applied hydrophilic treatment are provided, because a restraining force to align liquid crystal is exerted on the surface of each spacer 10a, a linear alignment defect region having close spacers as a nucleus as is denoted by numeral 15 may possibly occur due to an external force or stress, such as heat, in a portion where spacers are close to each other. Because there is no phase difference in this linear alignment defect region 15, it becomes a luminous spot in the black display.
Conversely, when a scattering density of spacers is reduced to control the linear alignment defects, the thickness of the liquid crystal layer becomes less uniform. The liquid crystal molecules in the liquid crystal layer then become unable to move stably, which results in a problem that liquid crystal display is disturbed.
An object of the invention is to provide a semi-transmissive liquid crystal display device using a reflection film provided with light transmission holes that is capable of performing a stable display by eliminating display irregularities caused by spacers mixed in the liquid crystal layer.
An object of the invention is to provide a liquid crystal display device capable of performing a stable display while suppressing linear alignment defects having spacers as a nucleus that occur between close spacers.